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Interferometry

OVERVIEW

The basic principles of interferometry date back to the seventeenth century. Interferometry uses the characteristics of electromagnetic waves to gather information about a sample. Interferometers have been employed to measure radiation from bodies in outer space as part of astronomical research. In the context of nanotechnology research, optical interferometers use light to gather information about the surface and properties of small samples. Optical interferometers generate two light beams, one is reflected off a reference surface and the other is reflected off the sample. The interference between the two light waves is measured to produce information on the sample’s surface characteristics.

White light interferometers and laser interferometers allow users to gather detailed information about the surface properties of a piece of material at very high levels of precision. Interferometers can indicate the flatness, roughness, and degree of curvature of surfaces. Interferometry is employed in optical (non-contact) profilers to determine film thickness and surface uniformity. Optical profilers are invaluable as quality assurance and failure analysis tools. Interferometers are popular due to their ease of use, versatility, and relatively low cost.

TYPICAL APPLICATIONS

ENVIRONMENTAL CHALLENGES

The mechanical properties of an interferometer require that the light or laser source, mirrors, stage, and detectors all maintain spatial uniformity to ensure accuracy and repeatability of results. Inherent building vibrations can sometimes be sufficient to frustrate accurate measurement. Interferometers are often employed in harsh environments, such as semiconductor fabs or production facilities. In these cases, high performance vibration isolation systems are a must.

Interferometers are also sensitive to ambient vibration sources such as street traffic, footfalls, and people talking. It is not uncommon to be able to watch fringes move and see data change due simply to people moving in the lab. In areas with street and foot traffic, it is recommended to employ supplementary vibration isolation systems to ensure repeatability of measurements and maintain throughput.

Interferometers are notoriously sensitive to low frequency vibrations. Thus, when one is used on the upper floors of a building it is advisable to employ active vibration control.